Electrical circuit



Nov. 3, 193a D. T. BELL 2959,19;

ELECTRICAL CIRCUIT Filed June 25, 1934 I lNl/LNTOR 0. 7 BELL ATTORNEY Patented Nov. 3, 1936 2,059,194 ELECTRICAL omourr Delamar T. Bell, West New Brighton, N. 2., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 23, 1934, Serial No. 732,043

Ill Claims.

This invention relates to protective devices and more particularly to such devices utilizing space discharge devices.

Heretofore, protective devices have been used in connection with amplifiers to avoid overloading the space discharge devices and associated apparatus. In some cases overloads have caused serious damage to the apparatus due to insufllcient power dissipation properties when the overloads have been continued for an appreciable length of time; That is, the prime consideration was the avoidance of continued overload rather than instantaneous overload. v

In present practice where power of the order of 1000 watts is impressed upon apparatus having mechanical as well as electrical functions, for example, loud-speakers, an instantaneous overload may damage the apparatus due to fail= ure of some of .the parts brought about by excessive mechanical stresses. The problem then is the protection from instantaneom overloads and from the nature oi" the problem, any arrangement requiring the actuation of a mechanical mass of appreciable inertia, such as a relay,

is not as effective as purely electrical arrangements having no inertia.

After a series of tests it has been determined that under some'circumstances a shunt arrangement associated with the main circuit produces the most desirable results since a series arrangement causes not only a continuous loss of energy, but also causes distortion in the currents passing through a series device and does not insure continuity of the circuit.

An object of the present invention is a pro tective device capable of substantially instantaneous operation;

Another object of the invention is the innprovement of amplifying arrangements.

A further object is the protection oi electro mechanical devices from electrical overloads.

A still further object is the protection of electromechanical devices from excessive mechanical of the shunt circuit which then absorbs and dissipates the excess energy.

The invention will be better understood from the following description and drawing forming apart thereof in which,

Fig. 1 is a circuit diagram by which the basic principle of the invention is explained;

Fig. 2 is a circuit diagram somewhat further advanced than that of Fig. 1;

Fig. 3 shows curves used in describing the cir- 10 cults of Figs. 1 and 2; and

i is a circuit diagram of an arrangement using two tubes in push-pull shunted across the main circuit to absorb energy above a fixed value.-

In general the requirements 0! a circuit-using 15 grid control discharge tubes is that the grid control voltage should have such a value that the discharge will occur at any desired anode voltage and that the discharge will cease immediately after the anode voltage falls below the extinguishing voltage.

In circuits heretofore used a source of constant direct current has been used for grid control.

Because of the characteristic of trigger tubes the grid voltage necessary to produce firing (that is, 25

discharge) at any definite anode voltage will vary with the temperature so thatlt is desirable to have the grid control circuit of such a character that the tube will discharge at approximately the same anode voltage regardless oi the m ambient temperature.

Referring now to Fig.3 along the axes of which e and :23 are measured grid and anode voltages respectively, curves A1 and A2 show the relation between anode and grid voltages for firing, at 35 two different temperatures. It will be noted that with constant grid voltage the anode voltage for firing is very dillerent. The ideal condition would be an arrangement where the grid voltage would change with temperaturean amount to insure m that discharge. occurs at a fixed anode voltage regardless of temperature change. This can be closely approximated by properly relating the grid and anode voltages in a manner to be hereinafter described. 5

In Fig. 1, windings 5 and. Z are the secondaries of a transformer, the primaries of which are con nected to an alternating, current source having a frequency in the audio frequency range. The source and primary windings are not shown.

Resistances R5 and. Bus are connected in series across winding 5. One end of winding 5 is connected to, the cathode 8 of a trigger type space discharge device having the cathode 0, grid 9 and anode I0. The other end of winding 5 is con- Fig. 1.

nected through resistance R5 to grid 9. One end of winding 1 is also connected to cathode I and the other end is connected throughresistance Rm to anode ll.

Resistance R5 is a linear resistance whereas resistance Rut is a non-linear resistance such as thyrite, that is, resistance Rm is such that as the voltage impressed across it increases its resistance decreases. The secondary windings are such that the voltage e1 across winding 5 is related to voltage ea across winding I as indicated by curve B oiFig. 3. with this relation between the voltages er and ea, the voltage e will be represented by curve C because of the non-linear resistance Rm. Also the voltage across resistance R5 will be represented by curve E since the drop across it must increase while (21 increases and that across Rm increases at a much slower rate. If the voltage across R5 is reversed and combined with the voltage e represented by curve C a combined curve F will result.

Such a reversal oi voltage as to resistance R5 and combination of the-two voltages may be had by the circuit of Fig. 2 where winding 6 is an additional secondary added to the arrangement of The grid voltage as will be seen from curve F increases negatively to a maximum value and then decreases so that the grid voltage characteristic cuts quite sharply across the anode firing curves as shown by curves A1 and A1. This means that with the arrangement shown in Fig. 2 the trigger tube'wiil fire at point when the tube temperature is that corresponding to curve A1 while point l2 gives the firing voltages when the tube is at a temperature corresponding to that of curve A2.

The impedance of the circuit described as a shunt to the main circuit with which it is to be used, is dependent upon the impedance of the anode circuit. That is, the amount of shunting from the main circuit will depend upon the impedance in the anode circuit of the trigger tubes. If a pure linearv resistance is used in the anode circuit of the discharge tubes, that is, if Rm in Figs. 1 and 2 is a pure resistance a definite shunt will be applied to the main circuit at the time the tube discharges and this shunt 'will remain constant during the entire period of discharge. However, if resistance Rm is a non-linear resistance, the value of which decreases with increasing voltage, then as the excess voltage increases, the impedance of the shunt circuit decreases and a greater portion of the excess energy is absorbed by the shunt circuit. .In other words, with a nonlinear resistance in the anode circuit the amount of shunting will depend upon the amount of overload. The use of a non-linear resistance in the anode circuit of the shunt circuit has another advantage which will be pointed out hereinafter.

For ideal operation the tube will discharge when the exciting voltage wave increases in the positive direction to the value of the firing voltage and discharge will cease the instant the exciting voltage falls below some definite low voltage which might be called the extinguishing voltage. :This extinguishing voltage is slightly less than the voltage between anode and cathode when conducting. However, an appreciable time is required for the tube to become deionized. Therefore, with a high frequency voltage causing the tube to discharge, it may happen that a succeeding cycle occurs before the tube becomes deionized and if in the succeeding cycle the voltage becomes greater than the extinguishing voltage, the discharge may be continued. This tendency to continuous operation is opposed by the non-linear resistance in the anode circuit since as the exciting voltage decreases the non-linear resistance increases greatly in resistance thereby obstructing the discharge current fiow.

The arrangement of the transformer windings is such that when the grid is at a positive potential with respect to the cathode the anode is at a negative potential and hence at a normally nonconducting part of the cycle. If this positive grid potential reaches a higher value than the extinguishing voltage before the tube is deionized, it will cause the discharge to continue, thus very materially shortening the time available for deionization. The" positive grid potential may even reach a suiiiciently high value to initiate ionization.

To prevent the grid voltage becoming positive with respect to the cathode it is necessary to connect in the grid circuit a unilateral transmitting device. A simple type of copper-oxide rectifier', although not a strictly unilateral transmitting device, has a preferential direction and the difference between the conduction in the two directions is sufilcient to. prevent. either initial ionization or continued ionization due to positive voltage in the grid circuit. It is not intended, however, to limit this function to a copper-oxide rectifier since any unilateral conducting device will accomplish the same purpose.

A circuit is shown in Fig. 4 which has performed very satisfactorily in a practical applica tion. In this figure the main circuit 20, which, for example, may be the conductors between two amplifiers, an amplifier and a loud-speaker or receiver or two stages of an amplifier, is shunted by the primary winding of a transformer 2|. This transformer is equipped with two secondary windings composed ofwindings 22, 23, 24 and .25 in one group and windings 28 and 21 in a second group. The midpoints of each of these groups is connected to ground at 28. In the case of the group 26, 21 the connection to ground is through a non-linear resistance Rms. Between each pair of windings 22, 23 and 24, 25 of the first group, there are inserted non-linear resistances Rmo and Run, respectively. Across each of these pairs of windings is connected a linear resistance Ru and R33, respectively.

In this circuit two trigger type tubes 34 and 35 are connected in push-pull arrangement. The cathodes of these tubes may be heated in any convenient manner preferably by alternating current from the ordinary mains of a light and power source through a transformer, not shown. The cathodes are grounded as shown or in any other convenient manner.

The grids of the tubes are connected through copper-oxide rectifiers 36 and 31 to the junction of winding 22 and non-linear resistance RNSD and the junction of winding 25 and non-linear resistance RNSI, respectively. Between the grid and cathode of tube 34 there is connected a resistance 38 and between the grid and cathode of tube 35 there is connected a resistance 39 to complete the rectifier circuit.

One end of each winding 26 and 21 is connected to the anode of tube 34 and tube 35, respectively.

As stated previously, the junction of the windings 26 and 21 is connected through non-linear resistance Run to the lead 0 connecting the cathodes to ground 28. It will be noted that the arrangement is such that current fiow occurs mainly in windings 26 and 21, while windings 22, 23, 24 and 25 are substantially only voltage windings because of the high resistances R32, R33, 38 and 39. The energy shunted from the main circuit is absorbed in the circuits associated with wind ings 28 and 21, while windings 22, 23, 24 and 25 govern the potentials applied to the grids. The similarity of this circuit to the circuits of Figs. 1 and 2 which show circuits for only one tube is readily apparent.

The general operation of the circuit of Fig. 4 for an overload in the main circuit is substantially as follows: As the voltage in winding 26 or the anode voltage of tube 34 increases in a positive direction, the grid voltage of this same tube increases in a negative direction as shown by curve F of Fig. 3. Ata certain anode voltage the grid voltage will start to decrease and at a predetermined point such as point I l or point l2 of Fig. 3 depending upon the tube temperature the grid voltage will reach the proper value for discharge with the anode voltage which obtains at the same instant. This discharge will be through resistance Rma so that during discharge there is in effect a resistance shunted across'the winding .28 of the transformer through tube which has a very low resistance when conducting. This shunt resistance is, of course, reflected through the transformer to the load so that discharge of one tube efiectively puts a shunt resistance directly across the load circuit'and the magnitude of this shunt is dependent upon the actual value of the resistance Rrm. When the anode voltage across tube 34 has passed its positive peak and starts decreasing the resistance RN22 becomes increasingly larger, so that the shunt on the load circuit is decreased as the voltage in winding 26 decreases. When the voltage in winding 26 has decreased to a value lower than the extinguishing voltage of tube 34, the shunt is substantially removed from the load circuit. The action. described for tube 34 of course occurs for tube 35 on the other half cycle'of the exciting voltage.

It is desired to point out that resistances Rim and R32 and also B15131 and R33 may be interchanged as to position in the circuit and the same result accomplished by reversing the polarity of each of the windings 22, 23, 24 and 25.

Whereas the invention has been illustrated and described as a protective device it has many other useful purposes, for example it may be used as a level indicator in which case a meter may be inserted in the anode circuit or the indication may be given merely by the glow of the tubes.

Whatis claimed is:

1. In combination, trigger type tubes arranged in pushq-pull relation, means for impressing upon the grids and anodes of said tubes voltages in opposing phases from the same source, said tubes tending to discharge at different values of impressed voltage as their operating temperature changes, non-linear resistances in the grid circuits so arranged that at a predetermined condition of grid and anode voltages, the tubes will become conducting regardless of tube temperature. 2. A combination in accordance with claim 1 characterized in this that means are included in the grid circuit to prevent the gridsattaining a positive voltage relative-to the cathodes.

' 3. A combination in accordance with claim 1 characterized in this that unilateral conducting devices are included in the grid-circuit.

4. A combination in accordance with claim 1 characterized in this that the impedance of the anode circuit during tube discharge period varies inversely as the anode voltage.

5. A combination in accordance with claim 1 characterized in this that a non-linear resistance is included in the common branch of the anode circuits.

6. A protective device to be shunted across a main ,circuit to protect the same against excess voltages comprising trigger type tubes arranged in push-pull relation, a transformer device to apply to the grid and anode of each tube voltages from the main circuit in opposite phases, two windings of said transformer separated by a nonlinear resistance adapted to apply grid voltages to said grids, whereby said tubes discharge at a predetermined condition of grid and anode voltages which remains substantially constant with varying tube temperature.

'7. In combination, a main circuit and a branch circuit shunted across said main circuit, said branch circuit comprising a transformer having a plurality of secondary windings, trigger type tubes associated with said transformer windings, one set of windings associated with the grids of said tubes, another set of windings associated with the anodes of said tubes, means for heating the cathodes of said tubes, means associated with the grid circuits of said tubes including non-linear resistances, which together with the arrangement'of the transformer windings provide a relationship between grid and anode voltages, both of which are derived from the transformer 'primary such that the tubes discharge always at a substantially fixed condition of grid and anode voltages whereby the impedance of the branch circuitis reduced and energy is shunted from the main circuit.

8. A peak voltage limiter for an alternating current line comprising a pair of gas filled tubes oppositely connected in shunt relation to said line and each having a grid control biased to pass current only in response to line voltage above a predetermined peak value and a nonlinear resistance in the common discharge circuit of said tubes, said tubes effectively isolating said resistance from said line circuit except during instants of tube discharge and in those instants effectively placing said resistance in shunt of said line.

9. In a circuit including a gas filled tube having a cathode, an anode and a grid or starting electrode, means applying voltages of reversed sign successively on both the anode and grid electrodes, the grid being driven negative when the 

